The present invention relates to an oil pump and, more particularly, to an oil pump of a type used as a hydraulic pressure generating source by a power steering device or the like for decreasing the force required to operate the steering wheel of a vehicle.
As an oil pump serving as a hydraulic pressure generating source for a hydraulic power steering device driven by a vehicle engine, a vane pump having a spool type flow control valve is generally known. A vane pump of this type has, in a housing space formed in its pump body, pump constituent elements comprised of a rotor, a cam ring, and a pressure plate and a side plate (or the inner surface portion of the pump body). The rotor has vanes. The cam ring houses the rotor to form a pump chamber. The pressure plate and side plate are arranged on the two sides of the rotor and cam ring to come into contact with each other. This pump constituent elements is placed in the housing space in the pump body. The rotor is axially supported by the inner end of an axially supported driving shaft extending from the outside of the pump body. Rotation of the engine is transmitted to the rotor to drive it.
When the rotor is rotatably driven by the driving shaft, the working fluid flows from the suction port of the pump to be taken into the pump chamber through a suction path formed in the pump body, and is sent from the discharge port to the discharge pressure chamber. The working fluid flows as hydraulic oil having a predetermined pressure from the discharge pressure chamber and is discharged from the discharge port through the discharge path. The spool type flow control valve is actuated when pressures before and after a restrictor formed on part of the discharge path are introduced to it.
When the flow control valve is actuated, a discharge fluid flowing in the discharge path is divided into an excessive fluid and a supply fluid which is to be supplied to the power steering device in accordance with the movement of the spool. The excessive fluid is connected to the suction side (or a tank) through a suction path and returned to it.
Generally, in most conventional spool type flow control valves of this type, the spool is disposed at a portion close to the outer surface of the pump body housing the pump constituent elements, to be displaceable in a direction perpendicular to the driving shaft (see Japanese Utility Model Laid-Open No. 5-96483 and Japanese Patent Laid-Open No. 8-291793).
In the vane pump described above, since the flow control valve is incorporated in the pump body at a portion close to the outer circumferential portion of the body and the spool actuates in a direction different from the axial direction of the pump driving shaft, it is difficult to make the entire pump compact.
In the conventional vane pump described above, when the engine operates at a high rotational speed, most fluid discharged from the pump chamber becomes excessive. Accordingly, the return path required to return the excessive fluid to the suction side with the flow control valve must have a large path diameter, increasing the size of the entire pump. The longer the path, the larger the path resistance produced by the return path described above, thus increasing the power loss of the pump.
Conventionally, an oil pump in which a flow control valve is arranged in the pump body to be movable in the axial direction is also known, as shown in, e.g., Japanese Patent Publication No. 52-10202.
In the oil pump of this type, as the flow control valve is provided on the extension of the axis of the pump driving shaft, the size of the pump in the axial direction increases. The entire structure including the path structure in the pump body becomes complicated to pose problems in terms of machinability and assembly of the respective portions as well.
What should be solved in the oil pump of this type is how to form a path structure in the pump efficiently, thereby improving the operation efficiency of the pump.
For example, in the conventional oil pump, when the flow rate of the discharge fluid discharged from the pump chamber reaches a predetermined value or more, the discharge fluid is partly returned as the excessive fluid to the pump suction side with the flow control valve formed at part of the pump discharge path. In the conventional oil pump, since the flow control valve is provided at a position remote from the pump chamber in the pump body, the return path required for returning the excessive fluid to the pump suction side becomes long. Since the return path has a small sectional area, a large path resistance acts on the excessive fluid. The large path resistance causes a large pressure loss of the excessive fluid. Since the fluid temperature (oil temperature) of the working fluid increases, the power loss in driving the power is large, leading to a low operation efficiency of the pump.
Of the discharge fluid discharged from the pump chamber, the excessive fluid is returned to the pump suction side with the flow control valve. To return the excessive fluid from the pump discharge side to the suction side, the path structure must be appropriately designed.
More specifically, when the rotational speed of the pump is low, the flow rate of the excessive fluid is small, and the flow velocity is also low. Even when the excessive fluid is merged with the suction fluid from the tank midway along the path, it is taken into the suction side of the pump chamber. At this time, the in-flow movement of the suction fluid and excessive fluid to the suction side of the pump chamber is not interfered with.
In contrast to this, when the rotational speed of the pump increases to reach a high speed, the flow rate of the excessive fluid from the pump discharge side increases in proportion to the rotational speed, and also the flow velocity increases. If the excessive fluid is merely merged with the suction fluid midway along the suction path, the flow of the suction fluid from the tank is interfered with at this merge portion by the jet of the excessive fluid. Then, the suction flow rate to the suction side of the pump chamber becomes insufficient to form a negative-pressure region, causing cavitation to likely generate noise. Any countermeasure is sought for to prevent this problem.